I clicked "Like" on this page of mine (using the Share Post plugin): http://www.calliejoscloset.com/shop/superhero-capes/
The first image on the page of the boy in the superman cape is the featured image for the page, so I expected that's the image it would use. Instead, it used this image, which isn't even on that page: http://www.calliejoscloset.com/wp-content/uploads/2011/04/c-roll-orange-blue-21-300x300.jpg
If I go to Facebook and paste the link I want to like (above) and looks at the thumbnail choices, it starts with the image I showed above as the first choice, and then shows me the actual images from the page as choices 2-4.
I tried other pages with similar results. My best guess is that it's pulling an image from the random image widget on the left of the page, but according to what the plugin is supposed to do - pick the featured image first - this shouldn't happen.